Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
Description of the Related Art
An electrophotographic image forming apparatus forms an electrostatic latent image by uniformly charging the surface of a photosensitive member by a charging device and exposing the charged photosensitive member by an exposure device. After that, the image forming apparatus forms a toner image by developing the electrostatic latent image with a developing material such as toner. As the charging device, for example, a scorotron charger including a casing, a grid (grid electrode), and a discharge wire (discharge electrode) is widely used. The scorotron charger can control the charged charges of the photosensitive member by a bias voltage (grid bias) to be applied to the grid, and has high controllability of the charging potential of the photosensitive member.
Japanese Patent Laid-Open No. 2010-44127 describes an image forming apparatus that can operate at a plurality of process speeds (rotation speeds of a photosensitive member), and includes a charger with a plurality of wires to obtain a discharge current necessary for each process speed. The image forming apparatus described in Japanese Patent Laid-Open No. 2010-44127 is provided with a high-voltage power supply for each of the plurality of wires of the charger to individually control a high voltage to be supplied to each wire. Furthermore, to control the charging potential of the photosensitive member to a potential appropriate for each process speed, the number of wires to be supplied with high voltages is switched in accordance with the process speed.
As described above, in the charger with the plurality of wires (discharge electrodes), a stray capacitance can be generated between two adjacent wires to form a current path. A ripple current flows through the current path due to ripple components contained in the output voltages of the high-voltage power supplies connected to the respective wires. The ripple current generated when the plurality of high-voltage power supplies operate may become large due to differences in the frequencies and phases between driving signals for respectively driving the plurality of high-voltage supplies. As the ripple current becomes larger, the surface temperature of a member of the charger, which serves as the current path, locally rises, thereby generating tracking phenomenon in the member. To prevent generation of such tracking phenomenon, it is necessary to perform insulation design, for example, increase the distance between the wires. However, this unwantedly prevents the apparatus from being downsized, and increases the apparatus cost.